Manufacturing of zinc oxide



Jan. 6, 1942. c. R. BERINGER MANUFACTURING OF ZINQO XIDE Filed se t. 26,1936 2 Sheets-Sheet? a wax-roe BY d 60m.

' ATI'ORNEYS Patented Jan. 6, 1942 Hungary, assignor, by mesneassignments,

Cornelius Raymond Beringer, Eugen Horvat, Ernst Horvat, Ferdinand Solt,and Alfred Spiegel, all of Budapest, Hungary 7 Application September 26,1936, Serial No; 102,75

3 Claims.

, The invention relaltestoimprovements in the manufacture of zinc oxide,particularly of-zinc white, from metallic zinc, e. g. crudezinc, and/orfrom metals bearing zinc, e. g. lead bearing zinc, and/or from metalalloys bearing zinc, etc.

. It is well known that in connection herewith 'dificulties result fromthe impurities, particularly from the lead contained in the metal bathto be evaporated because aportion of the impurities unavoidably passesinto the zinc vapour atmosphere, and the said fact-fundamentally impairsthat best quality of productwhich from a given smelting bath isobtainable at all. On the other side, the portion of the impuritieswhich does not passover into the zinc vapour atmosphere in the course ofthe progress of manufacture, gradually (in proportion to the evaporationof the zinc) enrich the metal bathin regard to itscorn tents ofimpurities, and in consequence thereof the; percentage of impuritiespassing over into the zinc vapour altmospherewill also graduallyincrease.

,The above facts result, altogether, in a fluctuation of the quality ofthe product obtained from oneand the same metal bath. The difli-.culties as mentioned above considerably increase when initial materialsof relative low purity are employed the use of which appears to be moreadvantageous owing to their lower price.

said fact, according to the above-mentioned earlier proposal of theinventor, the zinc bathis heated merely from above by radiated heatthrough effecting the combustion of the zinc Germany September 28, 19351 vapours in a closed chamber above the zinc bath itself so that thezinc vapour layer produced above the bath burns down continuously fromits upper side and is being continuously completed by supplementarysurface evaporation at its under side. Although this process has theadvantage that owing to the absence of any bottom heating there isnothingcapable to prevent the separation of the impurities by eliquationin a depression provided in the zinc bath container, 66

the absence of any bottom heating, however, results in the possibilityof heat being abstracted from the metal bath whereby the economy of thisprocess may possibly become impaired.

In accordance with another earlier proposal (see German patentspecification No. 444,863) the mploym nt t v l m u init marial is e d repossible b inserting in ser es a special reverberatory furnace beforethe containers in which the molten zinc is converted from the fluidcondition into the vapour condition, the said reverberatory furnaceoperating without any bottom heating in the manner of tub furnacesusually. employed for refining crude zinc. Thereby the contents ofimpurities is being diminished in the said reverberatoryfurnace by meansof eliquation toa degree low enough to be acceptable. This known processis extremely complicated and most expensive as regards working-expensesand also as regards the firstcosts of the plant'because the moltenmaterial prepared preliminarily in the reverberatory furnace for thepurpose of purifying the zinc has'to be conveyed from'the said refiningreverberatory furnace to .the other zinc bath containers and has to bepoured over in order to enable the zinc to be converted there intovapour and to be oxidized subsequently. In accordance with the presentinvention the conversion of the zinc from the fluid condition into thevapour-condition is effected in the same container in which theseparation of the im purities by eliquation is also performed so thatthe drawbacks resulting from the preliminary smelting and pouring-over(as mentioned above) are wholly dispensed with and, on the other side,

according to the invention, there is always provided a bottom heatingtooso'that the drawback of abstracting heat from the melting bath,resulting inprevious processes from the absence of bottom heating, islikewise wholly eliminated. According to the invention the :aboveobjectsmay be accomplished by performing the conversion of the moltenmaterial from thefluid condition, into the vapour condition by meansofbottom heating, or by means of bottom heating as-well as by means ofheating from above froma metal bath having a depressed extension whichduring the production of the zinc vapour, at least partially, butpreferably entirely, or inas great a degree as possible is exempted fromthe influence of-said bottom heating.

Accordingly, the invention is based on the idea that separating theimpurities by eliquationis possible also in case a bottomheating'lis'embe rendered more efiicient by exposing the abovementioneddepressed extension of the metal bath to a cooling effect. I prefer tolet the said cooling to act upon the said extension from all sides and,

furthermore, I prefer to bring about the said cooling by means of air.

It is possible to carry the said cooling effect in the bottom part ofthe depression down to a temperature lying between the melting points ofzinc and lead, respectively, so that the zinc is made to solidify in thelead etc. remaining still fluid in the bottom part of the depressionwhereby the zinc is enabled to float back again into the bath to thoseplaces which are exposed to the heating effect of said bottom heating.By these means it is possible to carry the separation of the impuritiesto a particularly high degree.

According to an exceedingly advantageous manner of carrying the processaccording to the invention into effect the metal bath is heated fromabove by means of radiated heat on its entire length whilst from thebottom itis heated along a portion of its length only and, at the sametime, the non-heated part of the bottom of the metal bath is shaped toform a pocket-like depression.

The method preferably employed in connection herewith consists inheating the metal bath from above and also from its bottom by means ofthe heat of combustion of the zinc in such way that the said heat ofcombustion of the zinc acts upon the metal bath from above along itsentire length and from the bottom but along a portion of its length.This may be accomplished in such way that the layer'of zinc vapourproduced above the bath is continuously being burned down from its upperside above the entire length of the metal bath itself and is, at thetime, utilized to heat only a fraction of the walls of the evaporationchamber said heated walls of the evaporation chamber being brought atleast partly (e. g. by the continuous or intermittent rotation of themelting kiln) underneath the metal bath and, at the same time, the saidwalls being shaped to form a pocket-like recess adapted to escape as faras possible the said heating effects of the heat of combustion of thezinc.

The utilisation of the heat of combustion of the zinc for providing abottom heating by producing the metal bath in a rotary tube kiln isknown (see for instance the German patent specification No. 557,499 ofthe inventor) and the provision of a depression within the said rotarytube kiln, for the purpose of depositing and collecting the impuritiesof greater specific weight, is also known. In the case of the said knownprocess, however, the effect of the radiated heat acting upon the bathsurface from above as well as the heating effect acting upon the bathfrom the bottom produced by the rotation of the kiln are both acting inthe metal bath along its entire length bebause the depression providedinside the rotary tube kiln is always heated by the combustion of thezinc to the same temperature to which the other internal parts of therotary tube kiln are heated. In fundamental contra' distinction theretoaccording to the present invention differences of temperature areintentionally produced at different parts of the bottom surface of therotary tube kiln. Thereby the removal of the lead etc. by eliquation isrendered possible and the said removal is more independent of the casualcircumstances of operation because, in the case of the process accordingto the present invention, both the difference of the specific weights ofthe various metals is utilized resulting in a separation merely bydeposition as well as the difference of temperatures at different placesof the bottom resulting in a separation of the various metals byeliquation too. As a result thereof the best quality of productobtainable from a melting bath having a certain degree of impurity isimproved as compared to the known processes whilst, at the same time,the economy of operation is also increased and, moreover, operation isrendered more indifferent to any variations of the degree of purity ofthe melting bath which fact finds expression not only in the increaseduniformity of the product obtained butv also in the fact that thedraining off of the impurities collected has to be effected at lessfrequent intervals.

With reference to the drawings which are of somewhat diagrammatic natureand for illustrative purposes only:

Fig. l is a side elevation in cross section and Fig. 2 is across-section through 2--2 of the rotary drum shown in Fig. 1; Figs. 3and 4 are each illustrating a further embodiment of a melting drumaccording to the invention.

According to Figs. 1 and 2, the metal bath is produced in a rotary tubekiln a and the combustion of the zinc vapours is being effected withinthe said rotary tube kiln a itself, for which purpose air and/or otheroxidizing gases (e. g. CO2) are introduced into the chamber a4 of saidkiln in such a manner that they pass over the layer of zinc vapour lyingabove the metal bath without coming into contact with the smeltingbathitself. Accordingly, the said layer will continuously burn down fromabove and will be continuously completed from underneath by the vapourscoming from said metal bath whereby the surface of said metal bath isheated by radiation from above and the walls a1, an of the rotarychamber or are heated directly too.

For initiating the process a grate furnace 0 may be employed and thegases of combustion of said furnace may possibly also supply theCOz-containing gas in case such gas is employed for the combustion ofthezinc in the chamber :14. It is preferably by rotating the kiln a thatthe heat of combustion of the zinc is utilized, in a manner per seknown, for providing also a bottom heating. For this purpose the saidkiln may be supported on rollers 7c as shown in such a manner that, bysaid rotating movement, the walls of the chamber al, which are heated bythe burning zinc vapours, get underneath the metal bath.

According to the invention the container for metal .bath is shaped onits bottom to form a depressed extension as constructed and arranged insuch a manner that the bottom surface aw of said extension as as well asits lateral walls should not be heated by the combustion taking place inthe evaporation chamber aior should be heated by the said combustion ina so small extent only that the said depressed extension as ispractically or in a so great extent as possible exempted from theeffects of the .bottom heating produced by rotating the kiln asexplained above. Accordingly, the walls of the chamber (Lila/11d, themolten metal contained in said chamber on the one-hand, and the walls ofthe extension as and the'material contained in said extension on the'other'hand, possessdiiferent temperatureswhere by,'as has alreadybeenexplained'theeliquation separating the impurities from the zinc 'isinfiuenced in a most advantageous manner.

I prefer to separate the internal spaceof the depressed extension asfrom the internal space a. of the drum a by one or a number of passagesa8 whereby the maintenance of a low'er tempera ture in the extension as,particularly at" the bottom portion at of the'latter' as compared to thetemperature of the bottom portion a3 is facilitated.

The internal space a; of the drum widened conically towards theextension as annularly surrounding the saiddrum so that the variousmetals capable of being separated bydeposition and eliquation willtendto movetowards the extension as already owing to theirliigherspecific weight.

The depressed extension as forms an annular flange as around the drum awhich flange is cooled on all sides by air whereby the temperaturedifference between the surfaces as and av is still further increased. Inconsequence'of this temperature difference very substantial internalstresses are set up in the material. of construction of said drum a sothat it may become necessary to stiffen the said flange as in a suitablemanner relatively to the shell surface of the drum a as indicated inFig. 1.

The depressed extensionae may be shaped according to any cross-sectionalshape .desired. I prefer to widen the said cross-section in the externaldirection, for instance in the manner of a truncated cone whereby thecooling effect within the range of the bottom surface at will stillfurther increase.

By giving a suitable shape to the extension as and/or by employingsuitable cooling I agents, the cooling effect may be increased so farthat in the extension as the zinc which possesses-a higher melting-pointwill solidify and, accordingly, will rise in thelead etc. which stillremains liquid thus returning again into the melting-bath located in thechamber a4.

All impurities, valuable in themselves -(lead, hard zinc etc.), collectin the extension as and may be removed, preferably each separately,-through one or more closable openings provided on the side walls and/or bottom of said extension as as indicated in Fig. 1.

The rotation of the drum a may be effected by any desired means, e. g.by means of a preferably reversible electric motor m, through the gearparts r2, 7'3, T1, or simply by means of rotating the rollers k, thedrum a being then carried along by these rollers through friction.

According to Fig. 1, the zinc fumes stream from the chamber as into aduct g and hence, for the purpose of their being condensed to zinc whitepigments, into a collecting device, etc. not shown. The supplementaryfeeding in of the zinc may be effected in any desired manner, e. g.through an opening it provided in the duct g and through the outletopening 1 for the zinc fumes. Preferably, also air and/or some otheroxidizing gas is introduced through the closable opening h, in order tocause a supplementary combustion of the Zinc behind the outletopening 1. That part.

of the duct g which joins on to the rotary drum kiln a may be maintainedat a high temperature to act as a storage chamber in which the zincfumes may stay for some while.

a maybe 1 'Accordingnto Fig. 1, the flange as surrounding the depressedextensionas ofthe smelting-bath is arranged at the outlet end of. thedrum a. In some cases it may be more advantageous to arrange the saidflange as in accordance with Fig. 3 in the centre ofthe length of thedrum and to widen the internal space a; in diameter towards the middleof the length of the drum conically or in the shape of a barrel.Thereby, the length of the paths leading to the depression as arediminished as compared to the arrangement according to Fig.- 1 and,thereby, the distribution of the internal stresses set up in thematerial of construction. by the intentionally produced differences oftemperature becomes also more favourable.

According to Fig, 4, depressed extensions as are provided in. annulararrangement on both ends of the drum a, and the internal space is inthis case widened from the middle of its length towards both ends. 1

Of course, more than two annular depressions as may be provided on thedrum a, and it is also possible to provide outside the-space a4 channelsconnecting the said depressionsas with each other and establishingcommunication between them. Furthermore, it would also be-possible toprovide an annular extension as surrounding the drum a continuouslyfollowing a helical or screw-like formation.

' According'to Fig. 1, the combustion of the zinc vapours is effected atleast partly in the smelting chamber a4 above the zinc bath. If desired,however, the zinc vapours may be led out from said chamber a; withoutcombusting them therein. In this case the combustion will take placeoutside the chamber a i, e. g. when the zinc vapours reach the duct 9,and the bottom heating which extends'but to a portionof the length ofthe smelting-bath must be effected by some other method known per se,for instance by electrical means. At the same time on the whole lengthof the smelting bath in any manner desired: a

' non-oxidizing heating by means of 'radiated'heat from above may bealso employed, for instance by means of hot non-oxidizing combustiongases streaming along above the layer of the zinc vapours produced fromsaid bath.

The process may be carried into effect in an oscillating kiln ofpreferably drum-like shape too,- in connection therewith, however, caremust be't'aken to prevent that the'melting-bath should 'be mixed by saidoscillation of the kiln soviolently' as to render the separation of themetallic impurities by eliquation and deposition impossible.Furthermore, the process may be effected in a stationary kiln too havingthe form of a channel or duct and being fitted with'any kind of a bottomheating, the said bottom being provided with a pocket-like depressionexempted in as great a degree as possible from the effect of said bottomheating. In this case also the combustion of the zinc vapours may beeffected in which the combustion of the zinc vapours takes place in theevaporation chamber (14 itself in order to utilize the heat ofcombustion of the zinc from above as radiated heat and possibly also asbottom heating by having the kiln rotated and/ or oscillated. Inconnection therewith I have noticed that the said heat of combustion ifit acts continuously without interruption may cause overheating, and inconsequence thereof the metal bath may get into a condition of violentlooiling, or may show a tendency towards the formation of bubbleswherebyas has already been mentioned previously the process ofseparating the impurities by eliquation and/or deposition is affecteddisadvantageously. In order to avoid the said overheating it isadvisable to introduce alternately a stream of gases of oxidizing nature(for instance air and/or CO2) and then gases of a reducing or othernon-oxidizing nature (for instance CO). Thereby the bottom heating ofthe metal bath is out out temporarily and during the periods ofintroduction of said neutral or reducing gases an opportunity is offeredto the metal bath for giving off any surplus heat content which may havebeen taken up by it during the time when oxidizing gases are streamingabove the layer of zinc vapours produced. The length of time of saidalternations has to be suited to the conditions of operation and dependson the degree of purity of the initial materials employed in each case.In a given case the stream of oxidizing gases may be cut out each timeafter a period of about 4 hours in order to replace it by a stream ofneutral and/or reducing gases for a period of about 15 minutes. By usingthe non-oxidizing ases for removing the zinc vapours from chamber (14and transporting them into special combustion chamber g, continuity inthe production of zinc white may be ensured.

The gases of a reducing or other non-oxidizing nature which are appliedintermediately or intermittently are admitted to the evaporation chamber114 preferably in a heated condition in order to avoid any too rapidcooling-down in the evaporation chamber. The said gases may be takenfrom a gas generator inserted in series directly before the evaporationchamber 114. so that the own inherent heat of said gases may be utilizedtoo.

The temporary replacement of the oxidizing atmosphere by a neutraland/or reducing atmosphere also offers advantages and may be employedaccording to the invention even in cases when the separation of theimpurities is effected according to the previously mentioned processesof the inventor because the boiling and bubbling of the bath alwaysimpairs the said separation of the impurities.

What I claim is:

1. A method of manufacturing zinc white pigments which consists inmelting in a chamber a metal containing zinc and impurities such aslead, passing a stream of oxidizing gas through the chamber above thesurface of the molten metal to oxidize the zinc vapors generated fromsaid molten metal and heating the bath in the chamber from above bymeans of the radiated heat produced by said oxidation to generatefurther zinc vapors, periodically interrupting the oxidation byeliminating the passage of the oxidizing gas and substituting therefor astream of non-oxidizing gas above the molten metal to thereby reduce thetemperature of the bath to a degree to prevent a violent boiling andbubbling of the bath, the zinc vapors generated during the flow ofnon-oxidizing gases being transferred by said fiow into a separatechamber where they are oxidized.

2. A method of manufacturing zinc white pigments which consists inmelting in a chamber having the form of a rotating drum a metal bathcontaining zinc and impurities such as lead, passing an oxidizing gas ina stream above the surface of the molten metal to oxidize the zincvapors generated from said metal while at the same time heating themetal by means of the heat produced bysaid oxidation in heating the wallof the drum, the rotation of the drum utilizing the heat of theoxidation throughout the wall of the drum for heating the metal,interrupting the passage of the oxidizing gas at predetermined periodsand substituting therefor a stream of non-oxidizing gas to thusmaterially reduce the heat of the wall of the revolving drum and of themetal to avoid that violent boiling and bubbling of the bath which wouldtend .to vaporize impurities in the bath for mingling with the zincvapor, and transferring the zinc vapors moved under the influence of thenon-oxidizing gas to an oxidizing chamber located beyond the drum.

3. A method as defined in claim 2, wherein the passage of thenon-oxidizing gases takes place at intervals of approximately threehours and is maintained for periods approximating twenty minutes.

CORNELIUS RAYMOND BERINGER.

